This invention relates to bar code scanners, and more particularly to a laser imaging system for generating a laser beam scan pattern with an extended depth of focus or working range.
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprising a series of adjacent bars and spaces of various widths. The bars and spaces having different lightreflecting characteristics.
A number of different bar code standards or symbologies exist. These symbologies include, for example, UPC/EAN, Code 128, Codabar, and Interleaved 2 of 5. The readers and scanning systems electro-optically decode each symbol to produce multiple alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470 and 4,460,120, all of which have been assigned to Symbol Technologies, Inc., the assignee of this application.
As disclosed in some of the above patents, one commonly-used example of such a scanning system functions by scanning the laser beam in a line across a symbol. The symbol, composed of alternating, rectangular, reflective and non-reflective segments of various widths, reflects a portion of this laser light. A photo detector then detects this reflected light and creates an electrical signal indicative of the intensity of the received light. The electronic circuitry or software of the scanning system decodes the electrical signal creating a digital representation of the data represented by the symbol scanned.
Typically, a scanner includes a light source such as a gas laser or semiconductor laser that generates a light beam. The use of semiconductor lasers as the light source in scanner systems is especially desirable because of their small size, low cost and low power requirements. The light beam is optically modified, typically by a lens, to form a beam spot of a certain size at a prescribed distance. It is preferred that the beam spot size be no larger than approximately the minimum width between regions of different light reflectivities, i.e., the bars and spaces of the symbol.
A scanner also includes a scanning component and a photo detector. The scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the scanner, or do both. The photodetector has a field of view which extends across and slightly past the symbol and functions to detect light reflected from the symbol. The analog electrical signal from the photodetector is first typically converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. This signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
With respect to the scanning system pattern generated by laser scanners, it has been previously known to generate the illuminating laser beam by collimating or focusing the beam using a lens system to create a beam spot of a given diameter at a prescribed working range. The intensity of the laser beam at this point, in a plane normal to the beam (i.e. parallel to the symbol), is ordinarily characterized by a "Gaussian" distribution with a high central peak. Therefore, at a given working range an intensely bright beam spot is generated upon and scanned across the bar code symbol. But, as the distance between the scanner and the symbol moves out of this working range, which is typically only a few inches in length, the intensity of the beam spot greatly decreases due to diffraction of the laser beam. Present scanning systems, accordingly, must be positioned within a relatively narrow range of distances from a symbol in order to properly read the symbol.